Photolithography employing a resist film is applied to fabricating electronic devices, such as semiconductor devices. A substrate is coated with a layer of resist material to form a photoresist thereon. The layer of resist material typically comprises a polymer with additives, such as radiation sensitizers, plasticizers, and adhesion promoters. The substrate can be a semiconductor wafer from which integrated circuit chips are formed or a module used to support and interconnect semiconductor integrated circuit chips.
The resist layer is exposed patternwise to electromagnetic radiation to change the solubility of portions of the resist layer. The resist layer is developed with solvents which remove the soluble portions of the resist layer leaving the substantially insoluble portions and uncovering parts of the substrate for further processing. The substrate is then typically subjected to etching or deposition processes.
There are two general types of resists, positive and negative. During the exposure step, the areas on a negative resist exposed to light undergo polymerization and change from being soluble to being substantially insoluble. In contrast, areas on a positive resist exposed to light undergo photosolubilization and change from being substantially insoluble to soluble. Positive resists are the resists of choice for fabrication area processing of state-of-theart circuits because they have a better resolution capability than negative resists. Accordingly, positive resists can resolve smaller openings due to the smaller size of the polymers therein. However, there are many devices with image sizes greater than 5 .mu.m for which negative resists can be used.
After a wafer has been aligned and exposed, it is subject to a development process. During the development process, an exposed photoresist film having the predetermined pattern is developed with a developer to remove resist film to form a predetermined pattern of photoresist film on the wafer. Referring to FIGS. 1a-1d. a conventional resist develop process consists of a developer dispense step (puddle forming step) as shown in FIG. 1a, a puddle develop step as shown in FIG. 1b, a water rinse step as shown in FIG. 1c, and a spin dry step as shown in FIG. 1d. As used herein, "puddling" refers to retaining a developer in a puddle by surface tension over the surface of a workpiece and "puddle develop" refers to a developing process using a developer puddle formed by puddling.
In developing an exposed resist film la formed on a semiconductor wafer 1, the wafer 1 is mounted on a vacuum wafer chuck 2 as shown in FIG. 1a. A developer 3 is spread in the developer dispense step by a nozzle 30 or the like over the surface of the resist film 1a to cover the surface of the wafer 1. The developer 3 is retained in a puddle by surface tension over the surface of the resist film 1a and the wafer 1 as shown in FIG. 1b.
After the puddle develop step, the wafer surface is rinsed with water as shown in FIG. 1c. In particular, the wafer chuck 2 holding the wafer 1 rotates while water 5 is dispensed from the nozzle 40. Immediately following the rinse step, the rotational speed of the wafer chuck 2 increases to a higher speed to dry the wafer 1 as shown in FIG. 1d. However, some of the resist which dissolves in the developer may precipitate during the water rinse step in water and harden and dry on the wafer surface because the resist is insoluble.
Consequently, a bridging problem often results since precipitant can stick to the wafer. Bridging is a condition where two patterns are connected by a thin layer of photoresist. Bridging can result from overexposure, a poor mask definition, or a resist film that is too thick. This type of defect has a substantial negative impact on the open/short yields for L/S (line/space) structures.
FIG. 2a shows an example of bridging which occurs when a positive resist and a clear (or light) field mask are used or when a negative resist and a dark field mask are used. The hatched regions in FIG. 2a represent a so-called "island", which are not removed during the develop process. The shaded regions represent precipitant which has bridged adjacent islands which causes short circuiting.
FIG. 2b shows an example of bridging which occurs when a positive resist and a dark field mask are used or when a negative resist and a clear field mask are used. The hatched regions in FIG. 2b represent a photomasking "hole" on the surface of the substrate. The shaded regions represent precipitant which has filled an area of a hole which can cause open circuits to form.
Resist bridging has generally taken place during prior art fabrication processes. However, other factors in the fabrication processes have tended to have a greater impact on product yield. As processing methods have improved and devices have become smaller and smaller, resist bridging has become an increasingly larger problem and an important consideration in determining how to increase yields. More particularly, with smaller devices resist bridging becomes a substantial contributor to reducing yield. Accordingly, there is a need in the art to substantially eliminate bridging which occurs in the resist develop process.